The present invention relates to a method and apparatus for obtaining substantially harmonic free periodic signals in an incremental or absolute measuring system.
A variety of photoelectric, magnetic, inductive and capacitive measuring systems are known to the art for measuring the relative position of two objects, such as the relative position between a slide piece and a bed of a machine tool for example.
In the case of prior art photoelectric length measuring systems, a measuring scale which defines a periodic graduation is mounted to a first object, and this scale is scanned by a scanning unit which is connected with a second object. The scanning unit includes for this purpose an illuminating system, and a scanning plate which may, for example, include two graduation fields. The graduations of these fields are typically offset with respect to one another by a phase angle of 90.degree. and correspond precisely in period with the graduation of the measuring scale. In addition, the scanning unit includes, for example, two photosensors, each of which is associated with a respective one of the graduation fields.
In the case of transmitted light measuring systems, the similar graduations on the scanning plate and the measuring scale are made up of alternating photopermeable and photoimpermeable strips which alternate along the longitudinal extent of the measuring scale (the measuring direction). The light flux generated by the illumination system passes through the graduations of the measuring scale and the scanning plate and then falls upon the photosensors. Thus, the light falling on the photosensors is modulated by the relative movement of the scanning plate with respect to the measuring scale. The two photosensors associated with respective ones of the graduation fields generate two periodic electric analog signals which are phase shifted with respect to one another by a phase angle of 90.degree. and are sinusoidal in wave form. These sinusoidal analog signals are applied as inputs to an evaluating arrangement which determines the measured position value from the analog signals. The period of the analog signals generated by the scanning unit is determined by the period of the graduation of the measuring scale. This period is in turn determined by the width of the alternating photopermeable and photoimpermeable strips along the measuring direction. In relative movement between the scanning unit and the measuring scale, each period of the graduation of the measuring scale results in the generation of a counting pulse which is counted and displayed.
In general, the periodic analog signals obtained from the graduations of a measuring scale in photoelectric, magnetic, inductive, and capacitive measuring systems are not precisely sinusoidal in wave form. Rather, in general such periodic analog signals include harmonic components as a consequence of inaccuracies in the graduations. Such inaccuracies can result, for example, from differing spacings between the photopermeable and the photoimpermeable strips of the graduations, or by an edge blurring of these strips.
One prior art approach to minimize the harmonic components in the analog scanning signal is to place high precision demands on the accuracy of the graduation. If the periodic analog signals are to be used to form exact position measuring values for each graduation period, and to provide improved measuring precision by subdividing the graduation periods of the graduation through the formation of interpolation values, the analog signals obtained from the graduation must be substantially free of harmonics. The prior art discloses several approaches to the formation of such interpolation values, such as the computer based approach described in West German DE-OS No. 27 29 697.
In addition to the measuring systems described above, other classes of prior art measuring systems generate triangular or trapezoidal analog measuring signals, which by their very nature include a large harmonic component.
German Pat. No. DE-PS 19 41 731 discloses a photoelectric length measuring system in which harmonic components in the analog signal generated in the scanning of the graduation of the measuring scale are reduced by means of a frequency filter diaphragm having a sinusoidal permeability characteristic. In this measuring system it is necessary for a special frequency filter diaphragm to be produced and installed on the measuring system. Moreover, this approach to the reduction of harmonic components is restricted to use in photoelectric measuring systems which operate according to the transmitted light measuring principle.